Aggregation of superparamagnetic colloids in magnetic fields: the quest for the equilibrium state
JS Andreu and J Camacho and J Faraudo, SOFT MATTER, 7, 2336-2339 (2011).
Previous experimental and simulation studies of superparamagnetic colloids in a strong external field have systematically shown a nonequilibrium aggregation process in which chains of particles steadily grow in the direction of the applied external field with the average length increasing as a power law over time. Here we show, by employing Langevin dynamics simulations, the existence of a different behavior under the effects of an external magnetic field: after a transient period of chain formation, the system attains an equilibrium state. Furthermore, a thermodynamic self-assembly theory supports the simulation results and it also predicts that the average chain length in the equilibrium state depends only on a dimensionless parameter combining the volume fraction of colloids phi(0) and the magnetic coupling parameter Gamma. The conditions under which this new behavior can be observed are discussed here.
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